Method for making hollow solid generated by rotation

ABSTRACT

The present invention relates to a hollow body of revolution (1), the wall of which comprises an internal first region (a) made, at least on its external periphery, of a single thermoplastic organic material A, and a second region having a first part (b), arranged so as to be continuous with the first region (a) and made of a thermoplastic material identical to the material A and of continuous glass yarns embedded in the said thermoplastic material, and a second part (c) forming the external periphery of the hollow body and made only of thermoplastic material. According to the invention, the wall has a volume void content Vv of less than 0.5%, preferably less than 0.2%.

The present invention relates to a hollow body of revolution based on athermoplastic organic material, in which continuous glass filaments,helically wound around its axis, are embedded.

Although the invention is not limited to such an application, it will bemore particularly described with reference to the manufacture of pipesof all types, especially those intended for conveying pressurizedfluids.

Another advantageous application is the manufacture of tanks intended tocontain fluids, especially pressurized fluids.

Plastics have already been widely used for this type of manufacture,but, in order to withstand the high pressures encountered, the wall ofthe pipes manufactured must necessarily have a very great thickness.This very great thickness gives the pipes a considerable weight.

In order to reduce the weight of such pipes, it has already beenproposed for them to be reinforced by means of reinforcing fibresarranged at their surface. Such a solution has been envisaged, forinstance, by Patent GB-A-2,077,880 which describes a composite pipeconsisting of an extruded former made of PVC or of polypropyleneimmediately covered, in the axial direction, with continuous glass yarnsimpregnated with a thermoplastic resin, helically wound around which arecontinuous glass yarns impregnated with a thermosetting resin, thelatter yarns themselves being covered with a layer of thermoplasticresin.

Apart from the fact that the continuous manufacture of these pipes isexpensive and very complex, the life of these pipes is very short. Thisis because the various aforementioned materials of which they arecomposed rapidly delaminate from each other, thereby rapidly reducingtheir burst strength.

This is why Patent EP-A-0,697,553 has proposed another type of compositepipe consisting of an extruded plastic former within which shortreinforcing fibres are dispersed, parallel to its axis, and around whichcontinuous reinforcing fibres are helically wound.

It turns out that the short fibres very rapidly become detached from theextruded former, thereby creating in the latter fracture initiatorswhich propagate rapidly. Consequently, the burst strength of the piperapidly decreases. To guarantee a minimum long-term burst strength,especially one that meets the standards in force, it is then necessaryto compensate for rapid propagation of fractures by again increasing thethickness of the extruded former, which results in a non-negligibleincrease in the weight of the pipe.

Furthermore, even if a minimum long-term burst strength is guaranteed,short fibres, once loosened, appear at the inner surface of the former.When the fluid intended to be conveyed by the pipe is water, thefood-related criteria that the pipe must meet are no longer satisfiedsince the visible short fibres run the risk of contaminating the saidfluid.

The object of the present invention is therefore to alleviate theaforementioned drawbacks and especially to propose a pipe of theaforementioned type which is lightweight and has an excellent long-termburst strength.

To do this, the subject of the invention is a hollow body of revolutionintended especially for containing a pressurized fluid, the wall of thebody comprising in its thickness:

a first region, the internal periphery of which is intended to be incontact with the fluid and at least the external periphery of which ismade of a single thermoplastic organic material A, and

a second region made of a thermoplastic material identical to thematerial A and of continuous glass yarns which are embedded in the saidthermoplastic material and are wound helically around the longitudinalaxis of the hollow body of revolution,

characterized in that

the second region comprises in its thickness a first part arranged so asto be continuous with the first region of the body and made of thethermoplastic material A and the glass yarns, and a second part formingthe external periphery of the body and made only of the thermoplasticmaterial A, and

the wall of the said body has a volume void content V_(v) of less than0.5%, preferably less than 0.2%.

It is specified that, within the context of the invention, the volumevoid content according to the invention may be measured in a knownmanner using a micrographic method of the image-analysis type. It istherefore appropriate to take several local measurements on the hollowbody and to carry out a statistical analysis thereof.

It is also possible to calculate the volume void content differently, ina generalized way using the following formula:

V _(v)=(d _(th) −d _(r))/d _(th)

in which d_(th) and d_(r) represent the theoretical density and theactual density of the body of revolution, respectively. The theoreticaldensity is calculated from the density of the glass and from the variousdensities of the thermoplastic organic material A weighted by theirrelative percentages. As regards the actual density, this is calculatedby taking the ratio of the actually measured mass of the body ofrevolution to the actual volume of the body.

The solution completely solves the problem posed. To achieve this, theinventors have firstly been able to analyse the key functions that acomposite pipe must fulfil and to demonstrate the shortcomings of thecomposite pipes according to the prior art, such as those mentioned inthe preamble.

According to this analysis, the reinforcing fibres must in principlewithstand all the hoop and axial stresses due to the pressure exerted bythe fluid flowing inside the pipe and the internal region made ofthermoplastic organic material must be impervious to and chemicallycompatible with this same fluid. A simple hooping of continuousreinforcing fibres over a former made of thermoplastic organic materialshould therefore fulfil these functions.

Now, the inventors have discovered that, in order to guarantee thedurability of a composite pipe of this type over time, it is absolutelynecessary for there to be both effective protection of the fibres andperfect adhesion between the various elements of which it is composed.

They have thus been able to devise a body, as claimed, with asufficiently intimate union between the various elements of which it iscomposed that a lifetime much longer than those encountered hitherto canbe guaranteed.

Furthermore, the invention makes it possible to obtain pipes which aremuch easier to transport and handle.

According to one advantageous characteristic of the invention, thethermoplastic organic material A may be a polyolefin, of thepolyethylene PE or polypropylene PP type, or else polyvinyl chloride(PVC). These materials have the advantages, among others, of beingchemically inert, of being able to withstand very low temperatures andof having a low manufacturing cost.

The density d of the thermoplastic organic material A may vary throughthe thickness of the wall and may, especially, be between 0.915 and0.960 g/cm³.

According to an advantageous variant, the continuous glass yarnsembedded in the thermoplastic organic material make an angle of between50 and 55° with the axis of the said body. Such an arrangement allowsthe axial and hoop resistance of the body to the pressure exerted by afluid flowing through or contained within it to be further increased,for the same amount of continuous glass yarns used.

According to another advantageous variant, the continuous glass yarnsembedded in the thermoplastic organic material make an angle close to90° with the axis of the body and other continuous glass yarns areembedded in the thermoplastic organic material, these being arrangedlongitudinally along the axis of the body.

The choice of one of these variants, of their combination or of anothervariant giving preference to a different angle of winding of thecontinuous glass yarns, as well as the choice of the respective amountsof the glass yarns in the direction in which they are placed, will bemade according to the specific constraints associated with eachapplication, such as the resistance to pressure, resistance toovalization, flexural strength, tensile strength, etc.

Preferably, the continuous glass yarns are distributed uniformly in thefirst part of the second region of the wall of the hollow body. Such adistribution of the yarns in the thermoplastic organic material veryfavourably enhances the mechanical properties of the body and guaranteesthem in the long term.

The invention also relates to a composite pipe consisting of the body ofrevolution defined above, coated with an external finishing andprotective layer made of thermoplastic organic material, preferably oneidentical to the material A.

The finishing layer according to the invention allows the pipe to bereliably protected against external attack likely to occur duringstorage, transportation, site operations and use.

The body or the pipe according to the invention are particularlysuitable for containing and/or conveying pressurized fluids.

The invention also relates to a process for manufacturing a hollow bodyof revolution, the wall of which is based on a thermoplastic organicmaterial A in which continuous glass yarns are embedded, these beingwound helically around the axis of the body. According to this process,the following steps are carried out in line:

a) a tape in the heated state is wound helically around a rotating tube,at least the external face of the wall of which is based on thethermoplastic material A, the tape being made of the same thermoplasticmaterial A and of continuous glass filaments embedded in this material;

b) part of the outer peripheral surface of the tube coated with the tapeis heated in a zone located immediately downstream of the zone where thetape comes into contact with the tube, to a temperature above themelting point of the material A;

c) local pressure is applied to that part of the outer peripheralsurface of the tube coated with the tape in a zone located immediatelydownstream of the heating zone of step b).

Advantageously, cooling is applied at the same time as the localpressure.

This process thus makes it possible to obtain a very good distributionof the glass filaments in the thermoplastic material by the use of acomposite tape rather than a composite yarn as used in PatentApplication EP 569,928.

Moreover, unlike the process of the invention which involves a heatingstep and a compression/cooling step downstream of the point where thetape is laid, the process of Application EP 569,928 reveals the need tosimultaneously compress and heat the composite yarn on the tube whenbrining the yarn into contact with the said tube in order to make iteasier for the turns of yarn to be bonded together and to expel the airbetween the filaments of the yarn, whereas, in the process of theinvention, the heating is downstream, allowing the thermoplasticmaterial of the tube and that of the tape, once the latter has been putinto place, to fuse together, and the pressure, also applied downstream,makes it possible to remove the air between the layers of the tape butnot at all between the glass filaments which are already contiguous, oneagainst another, with no air being present because of the very nature ofthe product covering the tube, that is the tape.

Furthermore, the process of the invention is particularly advantageousto apply to a thermoplastic material of the polyolefin type, since,although this material has a high viscosity, and is therefore difficultto process, especially in combination with fibres as is pointed out inApplication EP 569,928, the process succeeds perfectly in fusing thethermoplastic material of the tube with that of the composite materialof the tape covering it. It is all the more important to stress thisarrangement since polyolefin-type materials are advantageously of lowcost and compatible from the food standpoint.

Preferably, the tape arrives for step a) in the heated state, havingundergone, in a zone located near the tube, an operation of surfaceheating to a temperature above the softening temperature of the materialA but below its degradation temperature.

Also preferably, prior to step a), the following steps are also carriedout in line:

continuous comingled yarns, consisting of intimately mixed glassfilaments and filaments of material A in the form of at least one sheetof parallel yarns, are led in and assembled;

the said sheet is introduced into a zone where it is heated to atemperature between the melting point and the degradation temperature ofthe material A;

the heated sheet is made to pass through an impregnation device so as toobtain a densified and laminated tape of flatter shape than the tape asin step a);

the laminated tape is introduced into a zone where it is heated to atemperature between the melting point and the degradation temperature ofthe material A so as to obtain the heated tape as in step a).

According to this variant of the process, it is possible to use windingsof comingled yarns obtained in a direct process, such as that describedin Patents EP 0,367,661, WO 98/01751 and EP 0,599,695, which yarns havethe advantage of having an excellent comingling index, especially byvirtue of the stability of the process. Within the context of theinvention, the expression “excellent comingling index” should beunderstood to mean a comingling index whose mean value is less than 12.It should be pointed out that the mean value of the comingling index iscalculated in the following manner:

a number of cross sections are made along a given length of comingledyarn;

a grid is placed over each of these sections;

the surface distribution of the glass filaments and the filaments ofthermoplastic organic material is measured for each grid cell using amicrographic method of the image-analysis type;

for each section, the standard deviation of the surface distributions ofeach of the grid cells is calculated, this being the comingling index ofthe section in question;

the mean value of the comingling index for all the sections iscalculated.

This excellent comingling index results in an excellent distribution ofthe glass yarns in the thermoplastic material parallel to the axis ofthe body with the concomitant advantages such as those mentioned above.

Finally, the invention relates to an apparatus for implementing theprocess defined hereinabove. This apparatus is notable in that itcomprises:

means for at least surface heating a tape consisting of continuous glassyarns embedded in a thermoplastic organic material A, in a zone locatednear a tube, at least the external face of the wall of which is based onthe same material A, rotating about its axis;

means for helically winding the heated tape around the rotating tube;

means for heating part of the outer peripheral surface of the tubecoated with the tape, in a zone located immediately downstream of thezone where the tape comes into contact with the tube, to a temperatureabove the melting point of the material A;

means for applying local pressure to that part of the outer peripheralsurface of the tube coated with the tape, in a zone located immediatelydownstream of the heating zone.

According to a preferred variant, the means for surface heating the tapecomprise at least one infrared-type oven, preferably one operating withstrips or lamps regulated in terms of power according to the temperatureof the surface-heated tape. Such an oven has the advantage of being bothof high performance from the energy standpoint and of being easy toregulate.

Advantageously, the winding means comprise a laying head whichfacilitates the process of putting the heated tape according to theinvention in place. The laying head is able to rotate in a controlledmanner; preferably, it comprises three mutually parallel rollers ofhyperboloidal shape, the longitudinal axis of which is approximatelyperpendicular to the direction in which the tape runs, this itself beingparallel to the axis of pivoting of the laying head. Such aconfiguration for the laying head considerably improves the precisionand reproducibility with which the tape is put into place on the tube.

According to an additional characteristic, the means for heating thetube coated with the tape comprise a hot-air-blowing nozzle ofapproximately oblong cross section.

According to another characteristic, the means for applying thelocalized pressure comprise at least one rotating roller pressurized byan actuator.

When all the steps according to the invention are carried outcontinuously, using wound packages of continuous comingled yarnsobtained especially by a direct process, the apparatus may furthermorecomprise:

means for leading in and assembling continuous comingled yarnsconsisting of intimately mixed glass filaments and filaments of materialA in the form of at least one sheet of parallel yarns;

means for heating the said sheet to a temperature between the meltingpoint and the degradation temperature of the material A;

a device for impregnating the heated sheet so as to obtain a densifiedand laminated tape of flattened shape;

means for maintaining the tape at a temperature between the meltingpoint and the degradation temperature of the material A as far as themeans for winding the tape.

According to this variant, the winding and assembling means comprise acreel, from which packages of intimately mixed continuous comingledyarns consisting of glass filaments and filaments of material A areunwound, and at least one roller for guiding the comingled yarns.

Further details and advantageous features will emerge below from readingthe detailed description of an illustrative but non-limiting example ofthe invention, with reference to FIGS. 1a to 3 c which show,respectively:

FIG. 1a: an image of part of a hollow body according to the invention;

FIGS. 1b and 1 c: two micrographic sections showing the thickness of thewall of the hollow body according to FIG. 1a;

FIGS. 2a to 2 e: a general schematic representation of the apparatusused for manufacturing the hollow body according to FIG. 1a as well asthose of the various parts of this same apparatus;

FIGS. 3a to 3 c: images of hollow bodies, given by way of comparativeexamples.

FIG. 1a shows an image of the outer surface of part of a hollow body 1made of an organic thermoplastic material A, for example polyethylene,reinforced with continuous glass yarns according to the invention, theexternal diameter of which is 200 mm. This image shows the perfecthomogeneity of this part of the pipe over its entire height, with, inparticular, a uniform diamond-shaped pattern. This pattern ischaracteristic of one of the steps of the manufacturing processaccording to the invention and shows the angle at which the continuousglass yarns have been wound.

The weight of this part of the hollow body is approximately 4 kg/m.

FIGS. 1b and 1 c are two micrographic sections showing the thickness ofthe wall of this part of the hollow body, these being obtained using amicroscope. FIG. 1b shows the particular structure of part of the hollowbody. This part is divided, in the thickness direction, into:

an internal first region a, the internal periphery of which is intendedto be in contact with the fluid flowing or stored in the hollow body,this region comprising pure polyethylene with a mean density ofapproximately 0.955 g/cm³, the melt flow index (MFI) of which, at atemperature of 190° C. and for a weight of 5 kg, is 0.45;

a second region composed of polyethylene with a mean density ofapproximately 0.952 g/cm³, the melt flow index (MFI) of which, at atemperature of 190° C. and for a weight of 2.16 kg, is 18, and ofcontinuous glass yarns uniformly distributed along the axis and embeddedin the thickness of this region.

The second region is more particularly divided in its thicknessdirection, on the one hand, into a part b which is arranged so as to becontinuous with the first region a of the hollow body and comprisespolyethylene and the glass yarns and, on the other hand, into a part cconstituting the external periphery of the hollow body and comprisingonly polyethylene.

In fact, it is clearly apparent from the view in FIG. 1c, whichrepresents a local ×8 magnification of the previous figure, that thereis no visible “boundary” between the first region a and the part b ofthe second region. This absence of a “boundary” shows the perfectcontinuity of the thermoplastic organic material used. There wastherefore, during the process of manufacturing the hollow body, perfectunion between the two varieties of polyethylene of different densities,as will be explained below.

A measurement method, such as that of image analysis described above,makes it possible to quantify the volume void content of the wall ofthis part of the hollow body 1—it is about 0.2%.

FIG. 2a is a general schematic representation of the apparatus which wasused to manufacture the hollow body as in FIG. 1a. This apparatus 2firstly comprises a stand 3 inside which are fixed a motor and itssystem for synchronizing the various movements (not shown).

Projecting from this stand 3 is a mandrel 4 capable of being rotated,the cross section of which mandrel has been expanded by inflation sothat the extruded tube 5 made of PE 100 polyethylene, with a thicknessof 5 mm, is fitted over it with an interference fit. Also projectingfrom this stand 3 is a shaft 6, the rotation of which ensures that acarriage assembly 7 moves translationally parallel to the mandrel 4.

This carriage assembly 7 supports, respectively, a head 71 for layingthe constituent material of the second region of the hollow body, formedby the parts b and c, a press roller 72, the height of which is adjustedby a hydraulic actuator (not shown), a hot-air-blowing nozzle 73, aheating chamber 74 and a tensioning device 75.

Upstream of the carriage assembly 7 there is a creel 8 from which woundpackages of continuous comingled yarns 9, consisting of 800 glass yarnsand 800 polyethylene filaments, all intimately mixed, are unwound. Thesecomingled yarns 9 are sold under the brand name TWINTEX® by Vetrotex,the percentage weight ratio preferably being 60 in the case of the glassand 40 in the case of the polyethylene. Such a ratio makes it possible,when producing the hollow body, to obtain the best compromise between,respectively, the ease of processing and the mechanical performance bothin the longitudinal direction and the transverse direction.

The general operating principle of this apparatus 2 is given below.

The continuous comingled yarns coming from the unwound packages arefirstly assembled in the form of a sheet of mutually parallel yarns.This sheet passes through the tensioning device 75, which tensions it,and then into the heating chamber 74, which will be explained in detailbelow. On leaving this chamber 74, the sheet has been converted into atape 10, the polyethylene outer skin of which is still soft and has atemperature between 150 and 185° C. Next, the tape 10 runs through thelaying head 71 and emerges from the latter so as to be suitably appliedover the tube 5. The laying head is moved translationally parallel tothe longitudinal axis of the tube, while the latter is rotated so thatthe tape, and therefore the glass yarns, are wound helically around itat an advantageous angle of between 50 and 55° with respect to thelongitudinal axis, although this angle could also be close to 90°. Thewinding angle depends on the speed of movement of the carriage and onthe speed of rotation of the tube. In a zone located immediatelydownstream of the zone where the tape 10 and the tube 5 come intocontact, a hot-air-blowing nozzle 73 heats part of the outer peripheralsurface of the tube 5 coated with the tape 10 to a temperature between200 and 240° C. Immediately downstream of this heating zone, the pressroller 72 applies local pressure to that part of the outer peripheralsurface of the tube 5 which is coated with the tape 10 and heated, so asto secure the tape 10 to the tube 5. Since all the elements, beingmounted on the movable carriage 7, move parallel to the longitudinalaxis of the tube 5, as mentioned above, the latter becomes entirelycoated with the tape 10 by the to-and-fro movement of the carriage 7.This movement is obtained by the operation of the aforementionedsynchronization system.

The various parts of the apparatus will now be described in greaterdetail with reference to FIGS. 2b to 2 e.

The creel 8 (not shown) is essentially composed of a frame comprisinghorizontal rotating spindles supporting the packages of comingled yarnsaccording to the invention, these being sold under the brand nameTWINTEX®. These rotating spindles are braked by a suitable device, thefunction of which is to regulate the tension of the comingled yarns andprevent the packages from unwinding in an uncontrolled manner.Obviously, the creel 8 may be stationary or move with the movement ofthe carriage 7. This will depend in particular on the length of the tube5 to be coated according to the invention.

FIG. 2b is a detailed perspective view of the tensioning device 75 usedaccording to the invention. It comprises, respectively, in the directionindicated by the arrow f which gives the direction in which thecomingled yarns 9 run, a first eyelet plate 751, a second perforatedplate 752 parallel to the first, a grooved roller 753, two contactingwheels 754 and finally a tension feed 755. The eyelet plate 751 bringsthe comingled yarns 9 together and guides them at a shallow angletowards the perforated plate 752. The latter brings them together inpairs and aligns them with the roller 753, the grooves of which keep auniform spacing between the yarns 9 so as to obtain a homogeneous sheet91 after they have passed between the two contacting wheels 754. One ofthese two wheels 754 is provided with a speed sensor measuring the speedof the sheet 91, the contact between the wheels being such that thesheet 91 is driven through without slipping. As regards the tension feed755, this reduces the spreading of the comingled yarns 9, therebyavoiding any risk of the polyethylene yarns sticking to the walls of theheating device 74 which are raised to a high temperature.

As shown in FIG. 2c, the heating device 74 comprises two aligned ovens741, the heat sources of which are power-regulated infrared lamps 742,and between these ovens there is an impregnation device 743 which, byflattening the sheet 91, makes it possible to obtain a tape 10 byremoving, firstly, the air contained between the yarns, in order todensify the material, and, secondly, by completely embedding the glassfilaments in the thermoplastic material A. This step in the process ofthe invention plays an essential part in achieving the low void contentof the part b of the hollow body subsequently obtained. The ovens areregulated so that the temperatures reached by the tape 10 on leaving theoven 741 further upstream, that is to say the one closest to thetensioning device 75, are between 170 and 180° C., while those reachedon leaving the oven further downstream, that is to say the one closestto the laying head, are between 175 and 185° C. The impregnation device743 consists of three mutually parallel heating rolls 744 arranged in atriangular configuration. The height of the upper roll can be adjustedso that the tape 10 is flattened to a greater or lesser extent.

FIG. 2d shows the detailed structure of the laying head 71. This head 71comprises a rotary actuator 711 with adjustable stops, the rotation ofwhich drives, via a drive chain 712, a circular ball bearing 713fastened to three rollers 714 of hyperboloidal shape which can berotated about themselves. The rotation of the rotary actuator 711, whichis returned by two springs (not shown), makes it possible to make thehead pivot advantageously about an axis parallel to and approximatelycoincident with the mid-axis of the tape, in one direction or in theopposite direction, in order for the head to be inclined towards thedirection of linear movement of the carriage 7 so as to preventdistortion of the tape during winding, the angle of inclination giventhe head being tailored to the desired angle of helical winding of thetape. The rollers 714 are all heated and temperature-controlled so thatthe tape 10 is maintained at a sufficiently high temperature as itleaves the laying head 71. This advantageous structure allows the tapeto be laid very precisely and very reproducibly.

Finally, FIG. 2e shows the specific arrangement of the various elementsof the apparatus according to the invention in its downstream part.According to this arrangement, the oblong hot-air-blowing nozzle 73 isplaced perpendicular to the tube 5, being aligned with the laying head71 in a position diametrically opposite the latter. The press roller 72,the contact pressure of which, represented by the arrow P, is exertedvia a hydraulic actuator (not shown), is fastened to the movablecarriage and placed away from the laying head by a distance equivalentto approximately the width of the tape 10. This roller is cooled by afluid arriving via a hose 721. This arrangement of the various elementsat this point in the manufacture makes it possible to obtain, by fusingthe polyethylene of the tape 10 and that of the tube 5 together, perfectcontinuity between the polyethylene of the first region of the hollowbody and that of the part b of the second region, and makes it possiblefor a thin film of polyethylene to be exuded in the outer peripheralpart and enclose the glass yarns in the thickness of the hollow body, inorder to form the part c. This arrangement also allows the air betweenthe plies of the tape to be expelled. In particular, it is important forthe heating nozzle 73 and the press roller 72 to be placed downstream ofthe laying of the tape and therefore not to be employed at the momentwhen the tape comes into contact with the tube. This is because, sincethe laying head undergoes a to-and-fro movement along the length of thetube and therefore passes back into the same contact zone, if theheating nozzle were to act at the moment when the tape comes intocontact with the tube the latter would end up being heated, which wouldtherefore increase its diameter, and the final layers of the tape wouldthen be laid over a larger circumference; on cooling, since thethermoplastic contracts, the glass filaments would unfortunately nolonger be tight around the tube.

That part of the hollow body shown in FIG. 1a is obtained, afteroptimized adjustment of the parameters affecting the temperaturesreached during the process, and therefore the desired degree of unionaccording to the invention between the glass filaments and thethermoplastic material and between the various types of thermoplasticmaterial of different densities of the tube and of the taperespectively, the parameters being the speed of rotation of the mandrel4, the translational movement carried out by the movable carriage 7 byrotation of the mandrel, the flow rate and the temperature of the airblown by the nozzle 73, the distance between the tube 5 and the nozzle73 and the pressure exerted by the hydraulic actuator on the pressroller 72.

Table 1 below gives, by way of example, for a hollow body according tothe invention of external diameter equal to 200 mm, having atape-winding angle of 55° and with a thickness of the combined regions band c of 1.2 mm, the weight in kg/m, a total wall thickness without thefinishing layer in mm, the stiffness in kN/m² and the burst pressure inbar.

TABLE 1 Example according to the invention Weight (kg/m) 4.0 Totalthickness (mm) 6.2 Stiffness (kN/m²) 12 Burst pressure (bar) >70

In order to obtain an end-product of the pipe type serving for conveyingfluids, it is necessary for the external wall of this pipe to beguaranteed against any external attack; for this purpose, provision ismade to coat the hollow body of the invention with an external finishingand protective layer by extruding the layer, preferably usingpolyethylene.

It is accepted that after 50 years pure polyethylene retains less than50% of its initial characteristics. Moreover, it has been demonstratedthat the accelerated ageing tests, carried out by a technique well knownto manufacturers of polyethylene products, apply to the material of theinvention. Thus it has been demonstrated that such a pipe according tothe invention preserves high mechanical performance levels over time,retaining 60 to 80% of its initial characteristics.

Table 2 below is a comparison of the tensile strength of the materialafter 50 years between the pipe according to the invention and a pipemade of pure polyethylene.

TABLE 2 Tensile strength (MPa) Initial value After 50 years Pipeaccording to the 800 500 to 600 invention Pipe made of pure 20 10polyethylene

Finally, FIGS. 3a to 3 c show images of parts of the hollow body, thesebeing given by way of comparative examples:

the part of the hollow body in FIG. 3a was obtained using the samemanufacturing apparatus 2 as that just described, but from only packagesof continuous glass yarns;

the part of the hollow body in FIG. 3b was also obtained using the samemanufacturing apparatus 2, but from packages of continuous glass yarnsand of co-wound polyethylene yarns, but with no comingling of them;

finally, the part of the pipe in FIG. 3c was obtained using the samemanufacturing apparatus 2, from the same packages of comingled yarnsused according to the invention, namely those sold under the brand nameTWINTEX®, but without preheating the tape before it is deposited on thetube.

After analysing these images, it may be seen that none of these parts ofa hollow body exhibits the homogeneity of the part of a pipe accordingto the invention which is shown in FIG. 1a, this homogeneity beingcharacterized by perfect impregnation of the TWINTEX with thepolyethylene of the tube, and that, more specifically:

the use of only glass yarns does not allow the yarns to be impregnatedby the polyethylene of the tube 5, although the latter meltssuperficially;

the use of co-wound glass yarns and polyethylene yarns, but with nocomingling, results in poor impregnation of the glass yarns by thepolyethylene, this being revealed by a localized whitish colour, as wellas microscopic regions lacking in cohesion;

the fact of not preheating the tape does not result in a lack ofimpregnation, but in localized microscopic regions of the said materialor of the said tube lacking in cohesion.

Table 3 below illustrates a comparison of the burst pressures betweenthe various pipes shown in FIGS. 3a to 3 c and the pipe according to theinvention in FIG. 1a.

TABLE 3 Burst pressure (bar) FIG. 3a: glass alone 20 FIG. 3b: co-wound,but not 27 comingled, glass and PE FIG. 3c: TWINTEX, tape not 28preheated FIG. 1a: TWINTEX, with 40 preheated tape

It goes without saying that many modifications may be made withoutthereby departing from the scope of the invention.

What is claimed is:
 1. Process for manufacturing a hollow body ofrevolution, the wall of which is made of a material comprising athermoplastic organic material in which continuous glass yarns areembedded, said material being wound helically around the axis of saidbody, comprising carrying out the following steps in line: a) helicallywinding a tape in the heated state around a tube rotating about its axiswithout applying heat where the tape comes in contact with the tube, atleast the external face of the wall of which tube comprises thethermoplastic organic material, the tape comprising the thermoplasticmaterial and the continuous glass yarns; b) heating part of the outerperipheral surface of the tube coated with the tape in a zone locatedimmediately downstream of the zone where the tape comes into contactwith the tube, to a temperature above the melting point of thethermoplastic organic material; c) applying local pressure to that partof the outer peripheral surface of the tube coated with the tape in azone located immediately downstream of the heating zone of the step b).2. Process according to claim 1, comprising subjecting the tape prior toarrival for step a) in the heated state, in a zone located near thetube, to at least surface heating to a temperature above the softeningtemperature of the thermoplastic organic material.
 3. Process accordingto claim 1, comprising, prior to step a), carrying out the followingadditional steps in line: leading in and assembling continuous comingledyarns, consisting of intimately mixed glass filaments and filaments ofthe thermoplastic organic material in the form of at least one sheet ofparallel yarns; introducing the sheet into a zone where it is heated toa temperature between the melting point and the degradation temperatureof the thermoplastic organic material; causing the heated sheet to passthrough an impregnation device so as to obtain a densified and laminatedtape of flatter shape than the tape as in step a); introducing thelaminated tape into a zone where it is heated to a temperature betweenthe melting point and the degradation temperature of the thermoplasticorganic material so as to obtain the heated tape for step a).
 4. Processaccording to claim 1, wherein the thermoplastic organic material is apolyolefin or polyvinyl chloride.
 5. Process according to claim 4,wherein the thermoplastic organic material is polyethylene.
 6. Processaccording to claim 1, wherein the density d of the thermoplastic organicmaterial varies through the thickness of the wall.
 7. Process accordingto claim 1, wherein the density d of the thermoplastic organic materialis between 0.915 and 0.960 g/cm³.
 8. Process according to claim 1,wherein the continuous glass yarns make an angle of between 50 and 55°with the axis of the body.
 9. Process according to claim 1, whereincontinuous glass yarns embedded in the thermoplastic organic materialmake an angle close to 90° with the longitudinal axis of the body andwherein other continuous glass yarns are embedded in the thermoplasticorganic material, these being arranged longitudinally along the axis ofthe body.
 10. Process according to claim 1, wherein the wall of thehollow body comprises in its thickness: a first region (a), the internalperiphery of which is intended to be in contact with a fluid and atleast the external periphery of which is made of the thermoplasticorganic material, and a second region made of a thermoplastic materialidentical to the thermoplastic organic material and of continuous glassyarns which are embedded in the thermoplastic material and are woundhelically around the longitudinal axis of the hollow body of revolution,and wherein the second region comprises in its thickness a first part(b) arranged so as to be continuous with the first region (a) of thebody and made of the thermoplastic organic material and glass yarns, anda second part (c) forming the external periphery of the body and madeonly of the thermoplastic organic material, and wherein the wall of thesaid body has a volume void content V_(v) of less than 0.5%.
 11. Processaccording to claim 10, wherein the volume void content V_(v) is lessthan 0.2%.
 12. Process according to claim 10, wherein the continuousglass yarns are distributed uniformly in the first part (b) of thesecond region of the wall of the hollow body.
 13. Process according toclaim 1, which additionally comprises coating said hollow body ofrevolution with an external finishing and protective layer comprising athermoplastic organic material, thereby forming a composite pipe. 14.Process according to claim 13, wherein the thermoplastic organicmaterial in said external finishing and protective layer is identical tothe thermoplastic organic material in the hollow body.